I have this problem on my month old tank.
most likely brown diatoms which should go on its own as it is common on newly cycled tanks.
as your tank is older is there anything different you may have done recently that could of caused it such as using tap or distilled water to mix your saltwater instead of ro
Do you think your tank has been through a mini cycle for some reason?
If my brown diatoms are slow to clear I will have to introduce a couple of snails as the only cuc I have atm are a pair of skunk cleaner shrimp.